1. Field of the Invention
The present invention relates generally to a lithography technique for fabricating, for example, a semiconductor integrated circuit, a liquid crystal panel, and an image sensor and, more particularly, to a program storage medium and a method for determining the exposure condition and the mask pattern.
2. Description of the Related Art
With the recent advances in micropatterning of semiconductor devices, it has become difficult for an exposure apparatus to transfer (resolve) a desired pattern. Hence, to keep pace with the miniaturization of semiconductor devices, the exposure apparatus uses resolution enhanced technologies such as modified illumination and optical proximity correction (OPC) to optimize the mask pattern or the illumination configuration (effective light source distribution) used to illuminate a mask. The illumination configuration (effective light source distribution) is a light intensity distribution formed on the pupil plane of an illumination optical system, and also serves as the angular distribution of light which illuminates a mask. To optimize the illumination configuration, first, a layout pattern (target pattern) for a device, an evaluation position for a transfer pattern (optical image), and an evaluation value (for example, the size, the DOF, or the exposure margin) at the evaluation position are set. Next, the transfer pattern is calculated while changing the illumination configuration to obtain the evaluation value at the evaluation position on the transfer pattern. The transfer pattern calculation and the evaluation value obtaining are repeated until the obtained evaluation value reaches an allowable range or the number of changes in illumination configuration reaches a predetermined number. The illumination configuration is numerically represented by, for example, a function having inner σ and outer σ as its parameters (variables), which are optimized using, for example, the Monte Carlo method in annular illumination having a predetermined intensity. Even when the same mask pattern is used, the transfer pattern varies with a variation in illumination configuration, so the transfer pattern shifts from the target pattern upon changing the illumination configuration. Therefore, OPC is necessary to match the transfer pattern with the target pattern. OPC is done every time the illumination configuration is changed or the illumination configuration is changed by a predetermined amount.
U.S. Pat. No. 6,563,566 proposes a technique of setting a pattern to be formed on a substrate (wafer), and calculating the mask pattern and illumination configuration optimized by a mathematical approach. The technique disclosed in U.S. Pat. No. 6,563,566 analytically calculates solutions (mask pattern and illumination configuration) instead of repeated computation. Although the technique disclosed in U.S. Pat. No. 6,563,566 does not adopt OPC, the pattern to be formed on the substrate (for example, a wafer) and the optimized mask pattern are different from each other, so this technique can be said to be an illumination configuration optimization technique including mask pattern correction in a broad sense. The technique disclosed in U.S. Pat. No. 6,563,566 has a merit that it analytically calculates solutions, but requires not only limitation of the evaluation value to the tilt of an optical image but also limitation of the type of pattern to be formed on the substrate to one specific type. In this manner, the technique described in U.S. Pat. No. 6,563,566 is impractical because it has a demerit that it provides only a small number of degrees of freedom.
Unfortunately, it has become impossible for the prior art techniques relating to optimization of the mask pattern and the illumination configuration to form patterns that are rapidly becoming finer with sufficient accuracy. This is because the mask pattern and the illumination configuration are optimized separately, that is, they are not optimized simultaneously. As described above, OPC depends on the illumination configuration, and is therefore generally performed after the illumination configuration is determined (optimized). However, the mask pattern deforms upon OPC, so the illumination configuration determined before OPC may no longer be optimum after OPC.